Analysis of potentially mobile phosphorus in arable soils using solid state nuclear magnetic resonance
McDowell, Richard | Condron, L | Mahieu, N | Brookes, P | Poulton, P | Sharpley, AN
In many intensive agroecosystems continued inputs of phosphorus (P) over many years can significantly increase soil P concentrations and the risk of P loss to surface waters. For this study we used solid-state ³¹P nuclear magnetic resonance (NMR) spectroscopy, high-power decoupling with magic angle spinning (HPDec–MAS) NMR, and cross polarization with magic angle spinning (CP–MAS) NMR to determine the chemical nature of potentially mobile P associated with aluminum (Al) and calcium (Ca) in selected arable soils. Three soils with a range of bicarbonate-extractable Olsen P concentrations (40–102 mg P kg⁻¹) were obtained from a long-term field experiment on continuous root crops at Rothamsted, UK, established in 1843 (sampled 1958). This soil has a threshold or change point at 59 mg Olsen P kg⁻¹, above which potentially mobile P (as determined by extraction with water or 0.01 M CaCl₂) increases much more per unit increase in Olsen P than below this point. Results showed that CaCl₂ and water preferentially extracted Al-P and Ca-P forms, respectively, from the soils. Comparison among the different soils also indicated that potentially mobile P above the threshold was largely present as a combination of soluble and loosely adsorbed (protonated–cross polarized) P forms largely associated with Ca, such as monetite (CaHPO₄) and dicalcium phosphate dihydrate (CaHPO₄·2H₂O), and some Al-associated P as wavellite. The findings of this study demonstrate that solid-state NMR has the potential to provide accurate information on the chemical nature of soil P species and their potential mobility.
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